1. Field of the Invention
The invention concerns raised pavement markers that are retroreflective and are primarily used to delineate traffic lanes on roadways.
2. Description of the Related Art
Raised retroreflective pavement markers, as compared to stripes of retroreflective tape or paint, provide better long-range visibility at night, especially when wet, and when the tire of a vehicle strikes a raised pavement marker, the driver is alerted by the noise and vibration. Unfortunately, repeated tire impacts can damage and eventually break either the retroreflective material or the body of the pavement marker or tear the marker from the pavement.
Amerace Corp. markets as "Stimsonite 88" a raised pavement marker like that illustrated in U.S. Pat. No. 3,332,327 (Heenan). Its shell is a light-transmitting thermoplastic resin in the form of a truncated pyramid and has been injection molded to form on the inner face of one of its sloped sides a cube-corner pattern to provide an array of retroreflective elements. After coating that inner face with a light-reflecting material, e.g., aluminum, the plastic shell is "potted"with a relatively rigid filler material such as a filled epoxy resin, and the bottom surface of the cured epoxy resin is adhesively bonded to the pavement.
As is pointed out in the Heenan patent, the outer face of the cube-corner reflector (which the patent calls "the obverse face") is sloped from the roadway at an angle large enough for good reflectivity and small enough to allow adequate wiping by vehicle tires, i.e., from 15.degree. to 45.degree. and preferably 30.degree. to the surface of the roadway. The Heenan patent suggests "methyl methacrylate" for the shell of light-transmitting resin. Because polymethylmethacrylate resin is brittle, it has poor impact and breaks if flexed to a significant extent. For better impact resistance, the shell could be polycarbonate resin, but it has less resistance to abrasion than does methylmethacrylate resin.
While the "Stimsonite 88" pavement marker affords good visibility at night, it has poor daytime visibility, because substantially the only material visible to the approaching driver is the cube-corner reflector. Other raised pavement markers attain better daytime visibility by exposing a larger area of nonreflective surfaces to oncoming traffic. For example, see U.S. Pat. Nos. 3,392,639 (Heenan et al.); 4,208,090 (Heenan) 4,498,733 (Flanagan); 4,227,772 (Heenan); and 4,232,979 (Johnson et al.).
The bottom surface of a pavement marker of Heenan 4,227,772 is honeycombed by rectangular molding recesses that form vertical partitions or walls and serve to prevent sinks or shrink stresses during molding.
At the bottom surface of the Johnson pavement marker is an impact-absorbing material or pad which may be an elastomeric adhesive material such as butyl rubber by which the marker is bonded to the roadway (col. 7, lines 14-24). The flexing that would be permitted by such a pad is contrary to instructions accompanying most raised pavement markers which suggest that the adhesive should be rigid, because a flexible adhesive would allow the body of the pavement marker to flex under tire impact. Even a rigid adhesive cannot prevent the pavement from flexing, and when the pavement flexes, the body of the pavement marker necessarily flexes to the same extent. Repeated flexing due to impacts eventually can produce cracks in either or both the shell and filled body of a pavement marker and can also cause delamination between the shell and the filled body. Furthermore, flexing at the adhesive tends to fatigue the adhesive, thus permitting the pavement marker to loosen from the roadway. To minimize this, each raised pavement marker is sometimes mounted on a metal plate that has been fixed to a recess in the pavement, but to do so can be exceedingly expensive. Without using such a metal plate, pavement markers currently on the market can be expected to fail within two or three years and, in a typical installation, it can be necessary to replace about one-third of the markers every year.
Amerace Corp. markets as "Stimsonite 66" a raised pavement marker that is similar to that illustrated in the above-cited Flanagan patent. The "Stimsonite 66" pavement marker has an opaque synthetic resin body, at one surface of which is a plastic reflector that is retroreflective by virtue of a cube-corner pattern formed in its inner face. Also projecting from the inner face of the plastic reflector are septa (which the Flanagan patent calls "peripheral walls") that provide supports to prevent the apices of the cube-corner elements from contacting the opaque resin body of the pavement marker when impacted by a tire. The septa create a plurality of hermetically sealed cells beneath the reflector. The Flanagan patent explains: "In the event of damage to one or more of the reflector elements, only that particular air cell containing the reflective element loses its hermetic seal and thereby ultimately becomes optically ineffective" (col. 8, lines 25-29). Flanagan also says that the peripheral walls may be integrally formed as part of the support surface, or of the reflector, or as a separate piece.
The reflector of the "Stimsonite 66" pavement marker extends at an angle of 45.degree. to the roadway. As compared to the 30.degree. angle preferred by Heenan U.S. Pat. No. 3,332,327, this steeper angle reduces tire impact and also provides better retroreflective brightness. In the "Stimsonite 66" pavement marker, the overall thickness of the reflector is about 0.125 inch (3.2 mm) and there are 16 individual cube-corner elments per cm.sup.2.
U.S. Pat. No. 4,726,706 (Attar) also shows a raised pavement marker having a plastic cube-corner reflector that is formed with septa or load carrying walls. Because of these septa, the reflector of the Flanagan and Attar patents are not coated with aluminum or other light-reflecting material and thus avoid the loss of retroreflectivity that such a coating would entail. For further disclosure of the value of septa, see also U.S. Pat. No. 3,924,929 (Holmen).
Each of the plastic cube-corner reflectors of the above-discussed pavement markers is an individually molded piece and inevitably far from perfect optically when produced by molding at commerically useful production rates. The reflector of the raised pavement marker of U.S. Pat. No. 4,428,320 (Oplt et al.) can be cut from reflectorized sheeting. All that Oplt says about the sheeting is that it preferably is "a polycarbonate reflective tape, of the type manufactured by the Reflexite Corporation" under the trademark "Reflexite" (col. 2, lines 57-61). The "Reflexite" tape is understood to be made by forming a cube-corner replica by stamping, casting or extruding a thermoplastic resin onto the grooved surface of a master plate. The Oplt patent says that the tape is reflectorized, thus permitting it to be mounted flush against the body of the marker as illustrated. Although the Oplt patent does not say so, the "Reflexite" tape is poorly reflective except to light impinging substantially orthogonally, and it may be partly for this reason that the reflective tape of the illustrated marker extends almost vertical to the roadway. This also reduces tire impact.
A cube-corner reflector or sheeting that, unlike the "Reflexite" tape, does have good retroreflectivity along multiple viewing planes, is disclosed in U.S. Pat. No. 4,588,258 (Hoopman) and so would afford good retroreflectivity to oncoming traffic when mounted at a shallow angle to the roadway as in Heenan U.S. Pat. No. 3,332,327.
As compared to the reflector of the "Stimsonite 66" pavement marker, the cube-corners of the reflectors of the Hoopman patent are quite small, thus allowing the sheeting to be much thinner. In Example 3 of the Hoopman patent, the reflector was formed from 0.030-inch (0.75-mm) acrylic film, and it has about 1075 cube-corner elements per cm.sup.2.